1. Field of the Invention
The present invention relates to the field of power electronics. It relates specifically to a converter circuit for voltage maintenance in an electrical AC supply network as claimed in the precharacterizing clause of the independent claim.
2. Background Information
Conventional converter circuits, as are nowadays used in particular to compensate for dips in the network voltage of an electrical single-phase or polyphase AC supply network, in particular to compensate for short-circuits or load changes, are normally connected between the voltage source of one phase of the electrical AC supply network and an electrical load which is supplied by the voltage source. Such a converter circuit is specified, for example, in U.S. Pat. No. 5,329,222. In this document, an energy-storage capacitor is connected on the DC side to an inverter, with the energy-storage capacitor being used as an energy store and supplying the inverter. Furthermore, a supply device is provided, which is connected via an energy-storage capacitor on the DC side to the inverter and is used for supplying, in particular to provide an initial charge for, the energy-storage capacitor. The energy-storage capacitor has a high capacitance in order to allow the inverter to supply sufficient energy to compensate for a voltage dip to the electrical AC supply network, even in the event of a lengthy reduction in the network voltage. According to U.S. Pat. No. 5,329,222, the inverter is connected on the AC side to a secondary of a transformer. The primary of the transformer is connected to one phase of the electrical AC network.
A problem with the converter circuit according to U.S. Pat. No. 5,329,222 is that a transformer is required to supply electrical energy to compensate for a voltage dip in the network voltage of the electrical AC supply network and, owing to its high stray inductance, this transformer requires a correspondingly large amount of space, involves intensive installation, and thus results in considerable costs, in particular material costs. Furthermore, the energy-storage capacitor which is used as an energy store is, according to U.S. Pat. No. 5,329,222 in the form of a capacitor bank in order to allow a sufficiently large capacitance to be achieved. An energy-storage capacitor provided in such a way thus causes additional costs owing to its considerable space requirement and its installation and busbar construction.
The object of the invention is therefore to specify a converter circuit for voltage maintenance in an electrical AC supply network, which has a particularly simple and costeffective design, so that the installation and material costs of the converter circuit are minimized.
In the converter circuit according to the invention for voltage maintenance in an electrical AC supply network, an inverter is provided for each phase of the network, and is connected on the DC side via an energy-storage capacitor to a supply device. According to the invention, each inverter is connected on the AC side in series with the respective phase between a voltage source associated with that respective phase and an electrical load which is supplied from the voltage sources. This particularly advantageously means that there is no need for any transformer, connected on the AC side to the inverter, for supplying the respective phase, as a result of which the converter circuit is very simple, does not require intensive maintenance, and is thus cost-effective.
Furthermore, according to the invention, the supply device is connected on the AC side to at least two of the phases between the associated voltage sources and the respective inverters. In consequence, it is possible to select an energy-storage capacitor having a low capacitance, since the supply device which is connected to the phases can also draw electrical energy from the voltage sources for lengthy time periods in order to supply the energy-storage capacitor. This makes it possible to ensure in a particularly simple manner that even a lengthy reduction in the network voltage of the electrical AC supply network will be compensated for by the converter circuit according to the invention. Furthermore, the small size of the capacitance of the energy-storage capacitor which is now possible results in an advantageous reduction in the physical size, and thus in the material and busbar complexity.
In one preferred exemplary embodiment of the converter circuit according to the invention, the supply device in a polyphase AC supply network is connected on the AC side to each of the phases between the associated voltage sources and-the respective inverters. This also makes it possible for the phases to be utilized in a balanced manner in the event of a reduction in the network voltage in one of the phases or in a number of phases, by drawing electrical energy for the supply device from those phases which are not affected by the voltage dip. Furthermore, this measure advantageously makes it possible to supply the electrical load in a very largely stable and balanced manner, even if the reductions in the network voltage are not balanced between the phases.